Units should preferably be SI, being an international standard. The metric system as a fallback is acceptable, and it is preferable in situations where it’s more common (e.g. minutes, hours, days, months or years instead of seconds for long durations).
The rationale is simple: imperial units (or any other non-metric system) are not used outside the US and virtually not understood outside the US, the UK and perhaps Down Under. While the language on these boards is exclusively English, the audience is still international and imperial units should be considered too localised. For instance, I have no idea, not even a ballpark estimate, of how much a gallon is, and imagine that most people outside the US have the same problem.
SI, on the other hand, is an international standard and the standard for scientific communication (just like English). It should be universally understood, even in the US.
I propose the following guidelines:
- When quoting from elsewhere, preserve the original units, but supply a translation.
- For everything else, use SI or the metric system.
- Edit existing answers to supply SI or metric units, preserving the original author’s notation where necessary (see above).